Interleukin 4 (IL-4) is a member of a cytokine superfamily that exerts multiple effects on cells of hematopoietic origin. It was demonstrated that IL-4 induces a potent mitogenic response but not long-term growth in the IL-3-dependent myeloid progenitor cell line, FDC-P2. IL-4 and IL- 3 were shown to activate distinct tyrosine phosphorylation cascades in the same FDC-P2 cell background. This may reflect a difference in the biological function of these two cytokines. Insulin also induces a potent mitogenic response but not long-term growth in FDC-P2 cells. Test results demonstrated that IL-4 and insulin induced striking tyrosine phosphorylation of some 170 kD substrates, termed 4PS, in FDC-P2 cells, suggesting that these two factors share overlapping signaling pathways in hematopoietic cells.